The present invention relates to a fuel injector.
A fuel injector is described in German Patent No. 195-19-192 In the case of the translating device following from this document, a piezoelectric actuator is charged for opening the valve needle. In the process, the piezoelectric actuator expands and moves a primary piston against the force of a spring. Inside the primary piston, provision is made for a secondary piston fixedly joined to a valve needle. Inside the secondary piston, provision is made for a small spring arranged between an inner surface of the primary piston and the secondary piston. A working space filled with fuel is bordered by the primary piston and the secondary piston. In this context, the working space is designed such that a change in volume of the working space can only be achieved by a movement of the primary piston and/or of the secondary piston. Therefore, the motion of the primary piston gives rise to a motion of the secondary piston. The surfaces which are effectively available for displacing the volume during a movement of the pistons can be predetermined by the diameters of primary piston and secondary piston. The translation ensues from the proportion of these surfaces.
In this known translating device, the relatively complex and not optimized design in terms of compactness is a disadvantage. Moreover, the small spring provided in the secondary piston is not sufficient to actuate the fuel injector directly. In the mentioned document, therefore, the actuation of the fuel injector takes place via an additional amplifying device. This amplifying device requires relatively a high outlay and additional space.
It is a further disadvantage that a large mass is moved to actuate the valve via the actuator and that the action of the actuator takes place via elastic and/or resilient components and pressure fluids, resulting in relatively long switching times so that the fuel injector is not suitable as a quick-switching fuel injector for high switching frequencies.
The fuel injector according to the present invention has the advantage over the background art that the simple design approach of the translating device results in an inexpensive and considerably more compact construction. Moreover, no hydraulic medium is needed. Consequently, the new design approach is also suitable for gasoline fuel which has a tendency to form bubbles. Due to rigid lever segments running radially, the lift translation is moreover of high rigidity, thus permitting high switching frequencies which is why the fuel injector can also be used as quick-switching fuel injector.
Provision is advantageously made for a plurality of lever segments which are separated from each other by openings. This first of all results in a distribution of the load on the lever segments and, secondly, the lever segments are interconnected in an advantageous manner so that the arrangement of the individual lever segments relative to one another is definitively preselected and not defined by any additional fastening. The openings moreover prevent tensions in the lever segments, which has a favorable effect on the mode of operation of the lift translation.
The lever segments advantageously have embossings preferably running form the center point of the lever plate to the edge of the lever plate. In this manner, the lever segments are additionally stiffened, as a result of even shorter switching times are rendered possible.
The lever plate is expediently composed of two different kinds of segments of which one kind of segments is formed as rigid lever segments and the other kind of segments is designed as elastic, flexible spring segments. In addition to the advantages given by the rigid lever segments and already discussed above, further particular advantages are produced by the elastic, flexible spring segments since, due to the latter, the lever plate also assumes the function of the restoring spring or the pressure spring whereby components can be economized.
Advantageously, the lever segments are attached to a thin, elastic and flexible holding disk. In this manner, the rigid lever segments can be brought into a fixed, contact-free arrangement relative to each other. Moreover, this results in special possibilities of design for the lever segments of the lever plate, these possibilities of design not being possible or only with difficulty if the lever plate is manufactured from one piece. Advantageously, the lever plate is composed of a metal sheet or of plastic, in particular polyamide, thus resulting in a gasoline-resistant, inexpensive and compact construction of the lever plate.
If the fuel injector is an inward opening fuel injector, it is advantageous that the actuator acts upon the lever segments at a middle supporting location via a tubular thrust sleeve, that the lever segments engage on the valve body at an outer supporting location, and that the lever segments act upon the valve needle at an inner supporting location. In this manner, a particularly inexpensive, space-saving, and low-maintenance embodiment of the translating device is given by the lever plate. It is an advantage if the thrust sleeve is partially encircled by the actuator, the actuator having an opening for this. Because of this, the thrust sleeve can run inside the actuator, resulting in a particularly space-saving embodiment. Moreover, the interior of the thrust sleeve can be designed as a fuel line and/or offer space for additional components such as pressure springs, restoring springs, supporting and fastening elements. It is a further advantage that thrust sleeve is enlarged on the side of the lever segments. Given a predefined distance of the inner supporting location from the valve axis, the distance of the middle supporting location from the valve axis can consequently be adjusted, thus allowing the selection of a suitable lever ratio of the translating device, whereby the ratio of valve needle lift to actuator lift can be adjusted. Thus, in particular, it is possible to achieve a suitable lever ratio even for a thrust sleeve having a very small diameter.
It is an advantage that a restoring spring acts upon the lever segments and that a pressure spring acts upon the lever segments via the valve needle in a direction opposed to the pressure spring, the pressure spring exerting a greater torque on the lever segments than the restoring spring. In this manner, the opening force on the valve needle given by force of the pressure spring on the lever segments can be adjusted during an actuation of the valve, whereby the opening speed can be adjusted via the opening movement, allowing for the actuator movement and the valve needle mass. Moreover, the restoring force acting upon the valve needle can be predefined by the force of the restoring spring. In this manner, considering the movement of the actuator and the valve needle mass, the closing speed given by the closing movement of the valve needle can be adjusted. When the fuel injector is closed, the pressure spring is blocked by the thrust sleeve via the lever segments, as a result of which the torque of the restoring spring is sufficient to press the valve needle on the valve seat against the pressure in the fuel chamber so that a sealing seat is formed. In the case of this arrangement, the opening and closing of the fuel injector is controlled only indirectly via the actuation of the actuator. In this case, the forces acting upon the valve needle for opening and closing the valve are given by the pressure spring and the restoring spring. Because of the combination with lever segments of negligible weight and a small mass of the valve needle to be actuated it is possible to implement particularly high switching frequencies.
If the fuel injector is an outward opening fuel injector, it is advantageous that the actuator acts upon the lever segments at a middle supporting location at the thrust sleeve, that the lever segments engage on the valve body at an outer supporting location, that the lever segments act upon the valve needle via an inner supporting location, and that a pressure spring acts upon the lever segments, the torque with which the pressure spring acts upon the lever segments being inversely directed to the torque and the actuator acts upon the lever segments. The lift translation of an actuator lift of the actuator into a valve needle lift of the valve needle can be favorable selected in this manner. During the closing of the valve upon the completed actuation of the actuator, the advantage ensues that the valve needle, which is pressed against the lever segments by the restoring force of the restoring spring, does not experience any radial force at the inner supporting location and is consequently pressed against the valve seat in a distortion-free manner, possibly with the aid of suitable needle guides.